1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and particularly to a technique for writing multivalued data in a nonvolatile semiconductor memory device.
2. Description of the Background Art
In recent years, nonvolatile semiconductor memory devices such as a flash memory capable of nonvolatile data storage have become mainstream. A verify operation is performed in the nonvolatile semiconductor memory device such as a flash memory for determining whether writing was correctly performed or not.
For example, Japanese Patent Laying-Open No. 2001-312890 has disclosed the following verify operation. One page buffer P/B is connected to one bit line. Each page buffer P/B latches program data corresponding to a selected memory cell. The plurality of page buffers P/B are connected to sense amplifiers S/A·0 in a read/write circuit via column gates. In verify read, read data in a selected column is detected by using the sense amplifier provided for normal data reading. The read data detected by the sense amplifier S/A·0, i.e., a result of the verify read is transferred to the page buffer P/B in a selected column. The program data in page buffer P/B is rewritten based on a result of the verify read.
For writing multiple values into one memory cell transistor, two verify operations, i.e., lower foot verify and upper foot verify must be performed of determining whether a threshold of the memory cell is within a desired range or not. However, program data in a page buffer is rewritten with a result of the first verify read so that the second verify cannot performed.